Common processes for softening pretreatment of high-salt wastewater 1

High-salt wastewater usually contains high concentrations of calcium, magnesium, silicon ions and heavy metals, and needs to be softened pretreatment to reduce the water hardness to meet the requirements of subsequent membrane concentration or evaporation crystallization processes. The current mainstream softening technologies include chemical precipitation, membrane separation and ion exchange.

1. Lime-sodium carbonate combined softening process

In the primary reaction, lime (Ca(OH)₂) is added to adjust the wastewater pH to above 10, which promotes the formation of Mg(OH)₂ precipitation by magnesium ions, and at the same time generates hydroxyl compounds with heavy metal ions (such as Fe²⁺, Mn²⁺), and fluoride ions combine with calcium to form CaF₂ precipitation. Experimental data show that the removal rate of Mg²⁺ in the primary reaction can reach more than 95%, and the removal rate of fluoride exceeds 80%.

In the secondary reaction, sodium carbonate (Na₂CO₃) is added to further remove calcium ions through reaction. Secondary treatment can reduce the Ca²⁺ concentration to below 50mg/L, and the silicon content is significantly reduced due to the adsorption of Mg(OH)₂ flocs.

Limitations: Lime addition introduces a large amount of Ca²⁺, which requires an increase in the amount of sodium carbonate (about 1.5 times the stoichiometric ratio), and the cost of the reagent is high; the sludge output is large, and sludge dewatering equipment is required; the pH value needs to be adjusted frequently when the water quality fluctuates, and automatic control is difficult.

2. Liquid alkali-soda ash combined softening process

NaOH is used instead of lime as the primary reaction reagent to avoid the introduction of additional calcium ions. In a project, the initial Ca²⁺ of the desulfurization wastewater was 1200mg/L. After adjusting the pH to 11 with NaOH, the Mg²⁺ removal rate reached 98%; after the secondary addition of Na₂CO₃, the Ca²⁺ concentration dropped to below 80mg/L, and the sodium carbonate consumption was reduced by 30% compared with the lime method.

This method reduces the amount of sludge by 20%~40%; the process flow is simplified and is suitable for wastewater with a low calcium-magnesium ratio (Ca/Mg).

Limitations: The cost of NaOH is higher than that of lime (about 3 times), and the economic benefits need to be weighed; silicates may precipitate under high pH conditions, and dispersants need to be added to inhibit scaling.